Notes of turns & roads written in pen or pencil. A paper map if necessary for longer routes.
* Batteries never run down.
* If you wrap the notes in sticky tape, or book covering material, the ink won’t run.
Apart from that, I use a Lezyne Mega XL—fantastic battery life, esp. if you turn off things like cadence &c.
Carry a phone with thr route on Strava or similar for backup.